The present invention relates to the general field of producing fibrous sheets obtained by spreading several cables of fibers or yarns.
One favored, but nonexclusive field of application of the invention is that of the production of three-dimensional fibrous preforms intended for the manufacture of annular parts made from a carbon-carbon (C—C) composite material, in particular the manufacture of brake disks.
Brake disks made from composite material, in particular a composite material reinforced with carbon fibers and a carbon matrix, are well known. Their manufacture comprises the production of an annular fibrous preform and the densification thereof by a matrix.
One known method for producing an annular fibrous preform consists of producing annular fibrous sheets (i.e., a so-called transverse sheet and a so-called circumferential sheet) by spreading and juxtaposition of several cables of carbon fibers or carbon yarns. The transverse and circumferential sheets are then bonded to each other by needling and are rotated. A thick annular fibrous structure is obtained by bonding several layers to each other by circular needling.
This method, one example embodiment of which is described in document WO 2007/048946, thus makes it possible to obtain an annular fibrous preform directly from cables of carbon fibers or carbon yarns with practically no wastage.
With such a method, the cables forming the transverse sheet are more precisely laid alternating in one direction and the other between the outer and inner coaxial rings of an installation. Inevitably, such a laying causes tightening of the carbon fibers or yarns toward the inner ring (relative to the outer ring).
Consequently, to obtain an annular fibrous sheet that is completely homogenous over its entire width, it is necessary to compensate the increase in the surface density of the transverse sheet at the inner ring due to this tightening through a corresponding decrease in the surface density of the circumferential sheet in that location.
To give the circumferential sheet a decreasing surface density between the outer ring and the inner ring of the installation, it is possible to form said sheet with cables having a same width, but having decreasing weights between the rings. However, this solution is relatively impractical to implement and requires the use of cables with different weights.
Another solution consists of further spreading the cable(s) in question before they are laid. However, the known spreading solutions have a certain number of drawbacks. In particular, a spreading device by blowing air does not make it possible to freeze the width of the cable and thus prevent it from returning to its initial appearance before it was placed between the rings of the installation. It is also important for the spreading of the cable to be done while preserving the homogeneity of fibers within the cable. Lastly, it must be possible to spread the cable without creating tension in the cable.